The present invention regards an active load or source impedance synthesis apparatus. In particular an apparatus of the above type apt to the experimental characterization of radio frequency and microwave components and systems.
It is well known that for design and tuning of transistor and amplifiers, it""s necessary to obtain accurate information of the operation parameters thereof by measurements or, where possible, by theory.
Furthermore, the experimental characterization of new transistor it""s fundamental for the design of the other components to be linked with the transistor to obtain the desired overall amplifier performances.
The microwave (i.e. in the frequency range between about 500 MHz and about 110 GHz) transistor and amplifier characterization techniques can be divided into two main areas: linear characterization and non linear characterization.
The first case deals with the techniques to obtain the two-port network parameters under linear conditions, for example, the scattering parameters. This kind of measurements is based on vector network analyzer which are common instruments since two decades or more.
On the contrary for non linear characterization an established and consolidated method to measure the interesting parameters is still unavailable today.
A part from the traditional methods, based on the simple measurement of input and output power ratio or on the harmonic distortion measurement, in the last ten years a technique, called xe2x80x9cLoad/Source Pullxe2x80x9d, has being widely applied.
The xe2x80x9cLoad/Source Pullxe2x80x9d technique is based on the experimental measurement of the optimum load, i.e. of the source or load impedance (in the following we will refer to the load only for brevity) which gives the best value of some parameters (for example, the output power) of a particular device under test. The today commercially available load/source pull systems generally measure the output over input power ratio while varying the load/source impedance. FIG. 1 shows a scheme of a traditional load pull system with passive tuners.
The real time measurement of both the impedance and power levels is needed due to the non linear device conditions which prevents to obtain the optimum loading impedance through theoretical analysis from the S-parameters.
Due to its non linear behavior, a device under test E changes its output power with both the source and loading impedance, thus with only an experimental test set it is possible to measure the optimum loading which gives the proper matching for the desired performances.
Commercially available load/source pull systems are based on coaxial or waveguide mechanical tuners Tp to change the impedance shown to the transistor. A set (Sm) of directional coupler and bias tee is normally added between the transistor reference planes E and the tuners Tp to obtain the measurement signals.
The presence of both the intrinsic tuner loss and especially those of the measurement system prevents to obtain a unity or quasi unity reflection coefficient at the device reference planes, i.e. to obtain low or very low impedance values which are normally necessary to provide the conjugate matching of today transistors.
During the 80xe2x80x2, within universities and in some research labs, a new technique, called xe2x80x9cactive load pullxe2x80x9d, which overcomes the loss limitations has been developed. This technique provides for electronically synthetizing the source or load impedance shown at the device by substituting the mechanical tuner Tp with an active load loop.
The active load is made with a directional coupler Acc used to sample a signal proportional to the outgoing wave from the device E (see FIG. 2). After being filtered with a filter (Rf) this signal is properly controlled in magnitude and phase (through the variable attenuator Ra and the phase shifter Rs), then amplified and finally injected back towards the device under test. Thank to the amplifier, the reflection coefficient xcex93L, out of the active loop, can have a magnitude over than 1 (0 dB), which is then reduced to the unity at the device reference plane due to the losses.
Although the use of an amplifier allows to compensate for the measurement system Sm and probe Pr losses, which are between the active loop coupler Acc and the device reference plane A-Axe2x80x2, if the loss value is too high unwanted oscillations can start which compromise the use of the entire system.
This inconvenient prevents the use of the active loop technique especially over 20 GHz and up today a simple solution which overcomes the oscillation problem and gives reliable results at high frequencies has not been found.
The object of the invention is to solve the above drawbacks. In particular, the object is to provide an active loop impedance synthesis apparatus which does not show any parasitic oscillation even above 20 GHz.
This object is carried out through the apparatus described in the appended claims.
In particular, according to the invention, the loop directional coupler is arranged right before the device under test and after the main losses of the measurement system.
The present applicant, could demonstrate, after a theoretical analysis and several experiments, that the measurement system losses which were originally placed between the active load loop and the device under test dramatically affect the loop stability. With the new arrangement, according to the invention, the loss of the measurement system are included inside the loop and only lower the amplifier gain without compromising the stability.